1. Field of the Invention
This invention relates generally to technologies for switching and routing optical wavelengths. More particularly, this invention relates to innovative method, structures and processes to manufacture micro-electromechanical system (MEMS)-actuated, waveguide grating-based wavelength selective switches.
2. Description of the Related Art
Current state of the art in optical switching and signal transmission systems are limited to optical switching of an entire spectral range without wavelength differentiation or selection. Due to the lack of wavelength selection, an optical switch operation must frequently operate with a wavelength de-multiplexing and multiplexing device to achieve a purpose of transferring optical signals of different wavelengths to different ports. This requirement leads to more complicate system configurations, higher manufacture and maintenance costs, and lower system reliability. For this reason, even that optical switches provide an advantage that the optical signals are switched entirely in the optical domain without converting them into the electrical domain, the cost and size of application cannot be easily reduced. There is a strong demand to further improving the optical switches because optical switches are considered as critical enabling technology of optical-fiber networks. In the WDM networks of the past, adding, dropping or cross connecting of individual wavelengths has involved conversion of the signal back to the electrical domain. Development of all-optical switches for applications ranging from add-drop functionality to large-scale cross-connects is key to adding intelligence to the optical layer of the optical networking systems. However, with the current technical limitations, all fiber network systems implemented with optical switches are still quite expensive.
Due to the extremely wide transmission bandwidth provided by optical fiber, all-optical fiber networks are increasingly being used as backbones for global communication systems. To fully exploit the fiber bandwidth in such networks, wavelength-division multiplexing (WDM) and wavelength-division demultiplexing (WDD) technologies are employed so that an individual optical fiber can transmit several independent optical streams simultaneously, with the streams being distinguished by their center wavelengths. Since these optical streams are coupled and decoupled based on wavelength, wavelength selective devices are essential components in WDM communication networks. In the past, wavelength selective devices performed the adding, dropping and cross connecting of individual wavelengths by first converting the optical signal into the electrical domain. However, the development of all-optical WDM communication systems has necessitated the need for all-optical wavelength selective devices. It is desirable for such devices to exhibit the properties of low insertion loss, insensitivity to polarization, good spectral selectivity, and ease of manufacturing.
In today's all-optical Dense WDM (DWDM) networks, three prevailing types of wavelength selecting technology are used: (1) Thin Film Filter (TFF), (2) Arrayed Waveguide (AWG), and (3) Fiber Bragg Grating (FBG). Currently, TFF technology is the predominant choice when the spacing requirements of the wavelength selective device are greater than 100 GHz. The advantages of TFF-based devices are that they are relatively insensitive to temperature, have minimal cross talk, and provide good isolation between different wavelengths. However, devices built using current TFF technology have the following disadvantages: they are difficult to manufacture when the spacing requirement is below 200 GHz; the packaging cost is very high; and the yield is low. Due to these disadvantages, when the spacing requirements are 100 GHz and below, AWG and FBG wavelength selecting devices dominate the market. The advantages of AWG devices are they can support high channel counts, are easy to manufacture, and have a small silicon footprint. Meanwhile, the disadvantages are that AWG devices are prone to cross talk and their packaging is complex. FBG, the second dominant technology when the spacing requirements are 100 GHz and below, has the advantages of short development time, low capital investment, and low packaging cost. However, the FBG products available in the current market have relatively high power loss. Moreover, each channel requires a circulator, which increases component costs and possibly increases packaging costs.
Furthermore, there are several optical switching technologies under development today. These switching technologies are as follows: Micro Electro-Mechanical Systems (MEMS), Liquid Crystals, Thermal-Optics, Holograms, Acousto-Optic, etc. Among all these optical switching technologies, MEMS is emerging to be the most promising technology, as benefited from its potential of batch processing and cheap replication, as well as its sound-record on reliability in a wide range of applications. All the other technologies are still in the experimental stage and need years to become reliable enough for commercial applications. FIGS. 1A and 1B are functional block diagrams showing two alternate embodiments of MEMS optical switches. In FIG. 1A, the MEMS optical switch is implemented with a de-multiplexing device to first separate the input signals into multiple channels each having a specific central wavelength transmitted over a specific waveguide. Optical switching operations are performed for each of these de-multiplexed signals. Then a multiplexing device is employed to multiplex these switched signals into DWDM signals for transmission over optical fibers. FIG. 1B is a wavelength selective optical switch implemented with a de-multiplexing device to first separate the optical signal into channels of different wavelengths. The optical switching operations are carried out for each channel and these channels are connected to optical output ports. Again, a de-multiplexing operation must be performed first before wavelength selective switching can be carried out.
There are two types of optical MEMS switch architectures under development, or commercially available: mechanical and micro-fluidic. Mechanical-type MEMS-based switches use arrays of miniaturized mirrors fabricated on a single chip. The optical signal is reflected off this tiny mirror in order to change the transmission channel. Micro-fluidic-type MEMS-based switches, on the other hand, have no moving mirrors. Rather, they rely on the movement of bubbles in micro-machined channels.
Mechanical-type MEMS-based switches can be further classified into two catalogs according to mirror movement: two-dimensional (2-D) switches and three-dimensional (3-D) switches. In 2-D switches, the mirrors are only able to execute a two-position operation—that is, the mirrors can move either up and down or side by side. In 3-D switches, the mirrors can assume a variety of positions by swiveling in multiple angles and directions. These products (2-D switches or 3-D switches) are able to offer such benefits as excellent optical performance, minimal cross-talk, and the promise of improved integration, scalability, and reliability. On the other hand, these products and their methods of use are disadvantageous in the following aspects: first, in these switches, light travels through free space, which causes unbalanced power loss. Secondly, in order to steer each mirror, three to four electrodes need to be connected to each mirror, which is a major challenge to produce large-scale mechanical-type MEMS-based switches. Thirdly, alignment and packaging are difficult tasks particularly for large-scale switches.
While above-mentioned micro-mirror-based approach is widely taken by most major companies to build up their MEMS-based optical switches, Agilent Technology, Inc. has developed micro-fluidic-type MEMS-based switches by combining its micro-fluidics and ink-jet printing technology. In these switches, an index-matching fluid is used to switch wavelengths. This fluid enables transmission in a normal condition. To direct light from an input to another output, a thermal ink-jet element creates a bubble in the fluid in a trench located at the intersection between the input waveguide and the desired output waveguide, reflecting the light by total internal reflection. The advantages of these switches are that they have no moving mechanical parts and are polarization independent. The disadvantages of these devices are their reliability issues and the insertion loss issue for the large-scale switches.
A common drawback of both of these two types of MEMS-based switches is the requirement to work with external de-multiplexing and re-multiplexing devices in order to function properly in an optical networking system. The requirements of implementing de-multiplexing and re-multiplexing functions add tremendous complexities to the system configuration and significantly increase the cost of manufacture, system installation, and maintenance of the optical network systems. Another drawback of both of these two types of MEMS-based switches is that these prior-art switching systems are not wavelength selective switches. In another word, the switching systems cannot selectively switch a particular wavelength from an input waveguide to a desired output waveguide. In short, they are not wavelength intelligent devices. To add wavelength intelligence to optical switches, Bragg grating has been shown to have excellent wavelength selection characteristics. A Bragg grating behaves as a wavelength-selective filter, reflecting a narrow band of wavelengths while transmitting all other wavelengths. The Massachusetts Institute of Technology (MIT) has developed a technology for building Bragg grating devices in planar optical waveguides. These so-called integrated Bragg gratings offer many advantages over the fiber Bragg grating, according to MIT. However, since in order to switch the optical signal transmission requires a control of the routing of the optical transmission, an operational characteristic of wavelength selection alone is not sufficient to build an optical switch.
Therefore, a need still exists in the art to provide an innovative method for constructing MEMS-actuated highly integrated wavelength intelligent switches to add wavelength intelligence to the optical switches. It is desirable that the improved optical switch is able to eliminate unbalanced power loss, simplify fabrication and packaging processes, reduce the insertion loss and power consumption, and further improve the reliability of optical switches.